The move came a day after the South Korean navy fired warning shots to chase away two North Korean ships that had crossed a disputed western sea boundary.

Minor incidents are quite common on the western sea boundary, a line drawn by the American-led UN command at the end of the 1950-53 Korean War and which the North does not recognize. But this move comes as Pyongyang is pushing Seoul to accept leader Kim Jong-Un's proposal for talks aimed at easing cross-border tensions.

Animosities were touched off by Pyongyang's nuclear test in January and long-range rocket launch in February.

Reckless military provocation

The General Staff of North Korea's Korean People's Army said it would go ahead with "merciless retaliatory strikes," according to a statement released through state media.

The KPA said that the North Korean ships had been unarmed and were within the North's sea territory. It will now fire without warning at South Korean warships if they intrude into the North's waters by "even 0.001 millimeters."

"This reckless military provocation was evidently prompted by a premeditated sinister plot to bedevil the North-South relations and further aggravate the tension on the Korean Peninsula," the KPA said about Friday's incident.

The South responds

South Korea's Joint Chiefs of Staff issued a statement saying that the country's military had followed proper procedures to chase away the North Korean ships and called the KPA's claim of the response being a military provocation "ridiculous."

It said one navy ship issued an audible warning before firing five rounds of warning shots to repel a North Korean military vessel and a fishing boat that briefly crossed into South Korea-controlled waters on Friday morning.

Both sides complain of frequent incursions by the other and there were minor naval clashes in 1999, 2002 and 2009.

jbh/rc (AP, AFP)